1. Field of the Invention
This invention relates to a novel optically active compound useful in the fields of liquid crystal display elements and liquid crystal light-switching elements More particularly, it relates to an optically active compound bearing the spontaneous polarization of ferroelectric liquid crystal compositions, a liquid crystal composition containing the compound and a liquid crystal element using the same.
2. Description of the Related Art
At present, TN (Twisted Nematic) display mode has been most broadly employed as a liquid crystal display element. This TN display is provided with many advantages such as low driving voltage, low consumption of electric power, etc. However, as to its response rate, the display element is far inferior to emissive mode display elements such as cathodic ray tube, electroluminescence, plasma display, etc. A new type TN display element having the twist angle increased up to 180.degree. to 270.degree. has also been developed, but its response rate has still been inferior. As described above, although efforts for various improvements have been made, a TN display element having a quick response rate has not yet been realized.
However, in the case of a novel display mode using a ferroelectric liquid crystal, which mode has now been extensively researched, a notable improvement in the response rate has been expected (Clark et al, Applied Phys. lett., 36, 899 (1980)). This display mode utilizes a chiral smectic phase exhibiting a ferroelectricity such as chiral smectic C phase (hereinafter abbreviated to Sc* phase), etc. As liquid crystal phases exhibiting ferroelectricity, there are chiral smectic F, G, H, I, etc., besides Sc* phase.
Many characteristics are required for ferroelectric liquid crystal materials used for ferroelectric liquid crystal display elements for practical use.
Representative examples of the characteristics of ferroelectric liquid crystals are spontaneous polarization (Ps), tilt angle (.theta.), viscosity (.eta.), liquid crystal phase series, etc.
Molecules in ferroelectric liquid crystals can move only on a cone, and it is also possible to take two states wherein the direction of the major axis of molecules is vertical to the direction of the electric field. The angle formed between the two states has been called "cone angle", and half of the cone angle has been called "tilt angle (.theta.)". The ferroelectric liquid crystal display mode at present includes mainly two modes: a mode called birefringence mode using two upper and lower polarizing plates, and a guest-host (G.multidot.H) mode wherein the polarizing plate is single and a dichroic dyestuff is added to a liquid crystal composition. In order to give the best ratio of brightness to darkness (contrast ratio) of the two states, a tilt angle of 22.5.degree. is required for the birefringence mode, and that of 45.degree. is required for the G.multidot.H mode.
Further, since a relationship of .tau. oc .eta./Ps exists between the response time (.tau.) and Ps, .eta., in order to aim at a high speed response, a material having a large Ps value and a low viscosity is required.
Many characteristics are required for ferroelectric liquid crystal materials practically used for ferroelectric liquid crystal display elements, but it is the present status that no single compound can satisfy such requirements. Accordingly, mixtures of many materials have been provided therefor. Such ferroelectric liquid crystal compositions may be composed of not only liquid crystal compounds, but also non-liquid crystal compounds. Namely, there are a method of composing ferroelectric liquid crystal compositions from only ferroelectric liquid crystal compounds, and a method of mixing compounds or compositions exhibiting tilted smectic phases such as achiral smectic C, F, G, H, I or the like phase (hereinafter abbreviated to Sc or the like phase), as basic substances, with at least one ferroelectric liquid crystal compound or non-liquid crystalline, optically active compound to obtain a resultant exhibiting ferroelectric liquid crystal phase as a whole.
As the basic substances, compounds of various groups exhibiting achiral, smectic liquid crystal phases such as Sc, etc. have been used, but practically, liquid crystal compounds or liquid crystal compositions exhibiting smectic phases within a broad temperature range of low temperatures to room temperature or higher have been used. Among these smectic phases, Sc phase exhibits the highest response rate among ferroelectric liquid crystal phases. For this reason, Sc phase has generally constituted a liquid crystal phase of the basic substances. Examples of the components constituting these smectic C liquid crystal compositions are phenyl benzoates, Schiff's bases, biphenyls, phenylpyridines, phenylpyrimidines, etc.
Further, as compounds which are added to these basic substances to induce ferroelectricity, many compounds have so far been reported.
As the optically active materials which are added to ferroelectric liquid crystal compositions to induce ferroelectricity, the following lactone compounds A, B, C and D have been known:
Compound A: ##STR2## (Preprints of the 15th Japan Liquid Crystal Symposium, 1A05, p. 18 (1989)) (Japanese patent application laid-open No. Hei 1-199959),
Compound B: ##STR3## (Preprints of the 15th Japan Liquid Crystal Symposium, 1A11, p. 34),
Compound C: ##STR4## (Preprints of the 16th Japan Liquid Crystal Symposium, 1K117),
Compound D: ##STR5## (Preprints of the 16th Japan Liquid Crystal Symposium, 1K116).
These compounds have been described to have a large spontaneous polarization value and to be a superior optically active additive for ferroelectric liquid crystal compositions.
However, compounds A and B each have a linking group such as ester bond, ether bond or oxymethylene bond, between the lactone ring portion and the benzene ring or cyclohexane ring portion. From accumulated data of rotational viscosity in nematic liquid crystal compounds, it is evident that compounds having such a linking group increase viscosity to a large extent. Thus, the above compounds, too, can be said to raise the viscosity of ferroelectric liquid crystal compositions to a large extent.
Further, in the case of compounds C and D, while the five-membered ring lactone and the phenyl group are linked by a single bond, these compounds have a phenyl group linked to .alpha.-position of the lactone ring. Although such compounds having a phenyl group linked to .alpha.-position of the lactone ring have a relatively large spontaneous polarization value, such a value is not still sufficiently large.